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The instrumental bias (here expressed as bias*, the difference relative to San Carlos Olivine, SC-Ol) of oxygen isotopes in secondary ion mass spectrometry (SIMS) analyses of olivine and pyroxene were investigated. Sixteen olivine (Fo_0–100), nine orthopyroxene (En_70–100Wo_0–3), and three clinopyroxene (En_28-49Wo_45–50) reference materials (RMs) were utilized. The values of bias* are nearly invariant (within 0 ± 0.5‰) among magnesian olivine (Fo ≥60) RMs but decrease systematically with increasing Fe molar fraction down to ~ −10‰ (Fo₀). The session-to-session variability of bias* for Fo ≥60 olivines is ≤0.3‰ but becomes slightly larger for more Fe-rich compositions (≤1.5‰). Orthopyroxene RMs have a narrow range of bias* values (−1.7‰ to −2.6‰) that show session-to-session variability ≤0.3‰; correspondingly, the sputter rates are similarly low and resemble those of Fo ≥60 olivines …